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   难度：Easy
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   <h1 class="question_title">
    111. Minimum Depth of Binary Tree
   </h1>
   <p>
    Given a binary tree, find its minimum depth.
   </p>
   <p>
    The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.
   </p>
   <p>
    <strong>
     Note:
    </strong>
    &nbsp;A leaf is a node with no children.
   </p>
   <p>
    <strong>
     Example:
    </strong>
   </p>
   <p>
    Given binary tree
    <code>
     [3,9,20,null,null,15,7]
    </code>
    ,
   </p>
   <pre>
    3
   / \
  9  20
    /  \
   15   7</pre>
   <p>
    return its minimum&nbsp;depth = 2.
   </p>
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   <h1 class="question_title">
    111. 二叉树的最小深度
   </h1>
   <p>
    给定一个二叉树，找出其最小深度。
   </p>
   <p>
    最小深度是从根节点到最近叶子节点的最短路径上的节点数量。
   </p>
   <p>
    <strong>
     说明:
    </strong>
    &nbsp;叶子节点是指没有子节点的节点。
   </p>
   <p>
    <strong>
     示例:
    </strong>
   </p>
   <p>
    给定二叉树&nbsp;
    <code>
     [3,9,20,null,null,15,7]
    </code>
    ,
   </p>
   <pre>    3
   / \
  9  20
    /  \
   15   7</pre>
   <p>
    返回它的最小深度 &nbsp;2.
   </p>
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